A conventional method for delivering a cooling-lubricating spray during a machining process involves the application of a flooded coolant—typically an aqueous oil emulsion—under low to high pressure through a ported spindle and cutting tool. Most commercial CNC machining systems are specifically designed for flood and through-tool cooling-lubrication. However these systems present technical challenges with regards to adapting eco-friendly minimum quantity lubrication (MQL) and newer minimum quantity cooling lubrication (MQCL) schemes of the present invention. Challenges include pooling of oil droplets in transit within delivery channels and insufficient cooling provided by a carrier fluid such as air. Adapting internal MQCL spray jets within the machining system is also very challenging due to for example the inability to maintain the MQCL spray precisely pointed on the cutting zone following various tool change or part change operations.
Portable and open-air machining tools, for example portable drilling tools and knee mills, do not typically utilize flood coolant-lubricant schemes and rely heavily upon MQL and MQCL methods such as oil-air mist, cold air guns, and near-cryogenic sprays. Although easier to adapt MQCL technology to these platforms, there is a need for better MQCL technology to improve productivity and performance Moreover, adapting newer CO2-based MQCL schemes (referred to herein as Refrigerant 744 (R744 MQCL)) to through-ported spindle and jet spray coolant-lubricant schemes remains challenging. For clarity, the present invention uses the term “Open System” to describe R744 MQCL spray-at-tool schemes under ambient atmospheric conditions and “Closed System” to describe R744 MQCL spray-through-spindle and tool schemes under elevated internal air pressure conditions.
Bio-based machining oils and blends, for example Boelube® 70104, a proprietary blend of Oleyl and Cetyl alcohol developed by the Boeing Company, provide excellent lubrication for machining difficult materials such as titanium and carbon fiber reinforced polymer (CFRP), and sandwiched composites of same. However these lubricants do not provide appreciable cooling capacity to manage machining heat. Poor cooling constrains the productivity of the machining process in terms of speed, feed rate, and depth of cut (DOC). To mitigate this, oil, water, surfactant and other additives are used to formulate cooling-lubricating mixtures which are applied as a flood or mist coolant-lubricant into the cutting operation. However, use of such cooling-lubricating fluid sprays, even at the levels used in conventional MQCL processes, can be very messy and clean-up of surfaces following machining operations can be very challenging particularly where water-based MQCL is used.
This is particularly illustrated in machining applications involving portable drilling tools on aircraft production lines. In such applications, preferred cooling and lubricating (cooling-lubricating) schemes for MQCL typically involve small amounts of straight oil in air, cold air, or R744 MQCL sprays, for example as taught by the first named inventor in exemplary U.S. Pat. Nos. 5,725,154, 7,451,941, and 8,926,858.
Major drawbacks associated with conventional air-oil MQCL aerosol sprays include excessive fogging (aerosol formation) of the atmosphere and limited cooling capacity of air. Current air-oil MQCL schemes employ between 20 to 150 ml, or more, of coolant-lubricant per hour, which is a large amount of machining fluid for single point cutting and drilling applications. This results in messy machining and extensive post-machining surface clean-up operations. With regards prior art developed by the first named inventor, CO2-oil MQCL aerosol jet sprays have proven very effective as applied as composite jet sprays.
However conventional R744 MQCL schemes have proven quite challenging to adapt to closed systems (i.e., through-ported spindle and tool configurations), and particularly so providing consistent (and adaptive) cooling lubrication during machining operations where different cutting tools (having different sized coolant ports) and cutting conditions (experiencing changing cooling lubrication demands) occur during a machining process.
To illustrate this challenge, a through-ported machining spindle and cutting tool system can be conceptualized as a dynamic pressure reactor within which exists variable pressures and temperatures, fluids, and flow rates. The tool coolant port diameter determines the flow rate for the system (for a given pressure)—behaving as a changeable throttle for the machining fluid atmospheres introduced into the interior of the system (spindle and tool). Injecting and mixing variable amounts of CO2, oils and air have proven very problematic due to a multiplicity of tool port diameters and machining conditions, for which any change alters the dynamics of the system, and in particular CO2 phase transition and fluid mixing behaviors.
For example, optimal portable drilling conditions required to obtain optimal surface finishes and tool life for sandwiched composite fiber reinforced polymer (CFRP) and Titanium (Ti), termed stack-ups, requires changes in drilling speed and/or feed rate for the different materials. This capability is provided in portable drilling tools supplied by Apex Tool Group, Lexington, S.C., termed “Adaptive Drilling” technology, and is discussed under U.S. Pat. Nos. 8,277,154 and 8,317,437. However optimal machining conditions also require optimal drilling atmospheres—specifically cooling and lubrication chemistry, temperature, and pressure conditions. To date there is no effective and reliable solution for dynamically or adaptively monitoring, changing, and stabilizing R744 MQCL atmospheres within open or closed machining systems.
With reference to portable drilling machines using through-ported spindle and cutting tool systems, numerous technical constraints are experienced with conventional R744 MQCL schemes. These include the following:                1. Dry ice build-up within internal spindle or tool ports and cavities—difficult to precisely control mixing and internal pressures and temperatures during liquid CO2 injection.        2. Lubricant freezing and agglomeration into large masses prior to entering tool port—clogging, poor wetting, sputtering, and flow stoppage.        3. Inconsistent and variable cooling-lubrication effects, particularly during variable machining conditions characteristic of stacked composite machining        4. Poor spreading and wetting of frozen lubricant masses into and onto critical cutting interfaces and surfaces, respectively.        5. Excessive airborne oil aerosols emitting from cutting zone generated by rapid expansion of excessive quantities of CO2-oil mixtures (vaporization, sublimation, and gas expansion processes).        6. Oily residues are difficult to remove from complex machined surfaces due to high surface tension and thick film entrapment in complex surface features.        
The present inventors have determined through experimentation that these problems are caused by sporadic thermal and mass transfer instabilities during internal phase transitions—CO2 liquid-to-CO2 solid, CO2 solid-to-CO2 vapor, and Oil liquid-to-Oil solid (i.e., solidification). Dry ice particle build-up, particularly when formed as a CO2 particle and oil gel, is difficult to control once formed within the spindle cavity and/or cutting tool coolant ports. This condition is exacerbated under low internal spindle-tool cavity pressure and temperature conditions. Microscopic frozen oil/dry ice particles once formed lack thermal conductivity which prevents fast melting (and sublimation) and efficient flow through internal passages. Larger agglomerations of frozen and entrapped oil and CO2 particles worsen the situation. As such, it is very difficult to precisely control internal phase transitions and resulting pressure, temperature, and flow instabilities using conventional means and is particularly difficult at low internal operating air pressures normally used with conventional spindle-tool R744 MQCL schemes. Dynamic changes in tool coolant port diameters, behaving as system flow throttles, and variable cutting conditions (i.e., thermal loads) exacerbate the problems thus described.
To overcome these problems, higher pressure cooling and lubricating schemes have been developed. For example, the first named inventor has developed an internal overpressure scheme (i.e., 100 psi and higher) using for example a CO2 gas atmosphere within the through-ported spindle and cutting tool, into which various quantities of solid CO2 and oil are injected and mixed. This approach does improve performance but does not completely resolve oil gelation problems. Moreover, high pressure conditions can be problematic and dangerous for machine tool operators, and particularly with portable drilling equipment. Still moreover, higher internal pressures require bulky high pressure lines and cause premature rotary seal wear and failure. Higher internal pressures utilize significantly more fluids (i.e., air and CO2). In addition, machining chips and debris are ejected from the cutting zone at higher velocities which requires additional safety measures.
In a related approach in the prior art, high pressure liquid and supercritical CO2 spray-through-spindle and tool configurations have been developed, available from Fusion Coolant Systems and described under U.S. Pat. Nos. 7,414,015 and 8,167,092. However these alternative high pressure CO2 cooling-lubrication schemes are expensive to implement and use, pose high pressure hazards to workers, and are not easily adapted to through-ported machining systems without extensive redesign of coolant channel systems, spindles and tool changers.
Besides conventional oil-in-air (and cold air) MQL schemes, other conventional MQCL solutions involve the application of evaporative mixtures of highly volatile solvents (with high latent heats of vaporization) and lubricating oils to provide both cooling and lubrication actions. One such example is U.S. Pat. No. 6,326,338, which teaches the use of n-propyl bromide-oil mixtures. Major drawbacks of the '338 approach are that n-propyl bromide is toxic to humans and possesses a very low flash point of 22 deg. C., thereby not suitable for total loss systems in which the coolant-lubricant spray ultimately enters the ambient atmosphere. Frictional heating and sparking common to many machining operations poses an ignition hazard as well. In another example, Vertrel® XF is a fluorocarbon solvent offered by DuPont which can be blended with lubricating oils to provide an evaporative cooling and lubricating mixture. Although non-toxic and non-flammable, the Vertrel® process is too expensive to use in a total loss MQCL machining application. Moreover, the use of many of these conventional volatile solvent-oil cooling-lubricant schemes in productive machining applications are not permitted by manufacturers due to a combination of employee exposure and environmental quality concerns.
Finally, so-called green solvents are attractive for many industrial processes due to their low melt point, high boiling point, high solvent power, human safety, eco-friendliness, and renewability. However, most are not suitable as carriers for lubricants because of limited oil solubility, high boiling points, and/or material compatibility issues. For example, propylene carbonate (PC) is not suitable for use as an evaporative lubricant carrier solvent within a machining process due to chemical, physical, and material compatibility constraints. PC is hygroscopic and possesses a highly polar cohesion parameter of 27 MPa1/2 with minimal bio-based oil solubility. In addition, PC possesses a very high surface tension—41 dynes/cm—which decreases wettability and requires the addition of a surfactant to lower fluid surface tension for machining applications involving for example low surface energy and non-polar carbon fiber reinforced polymer (CFRP), with a surface energy of approximately 58 dynes/cm. More significantly and with respect to the present invention, PC has a very low evaporation rate of <0.005 (Butyl Acetate=1), making it unsuitable for thin film oil deposition. PC is non-volatile and poses an entrapment issue, for example in complex machining applications involving aircraft fuselage and wing stack-ups comprising sandwiched panels of CFRP, Titanium, and Aluminum. Finally, many green solvents are not compatible with certain substrates and pose fire hazards. For example, non-toxic and generally eco-friendly solvents such as acetone and methyl ethyl ketone (MEK) are incompatible with paints, adhesives and sealants and exhibit high flammability.
As such there is a present need to provide an improved R744 MQCL system that provides a precise low-temperature cooling lubrication process absent of the constraints thus described. Moreover an improved R744 MQCL system is needed that minimizes post-machining surface clean-up operations. Finally, an improved R744 MQCL system is needed that can perform better than the conventional approaches; a system that is adaptable to a variety of machining systems and tools, particularly closed-systems such as those found in portable drilling machines; and a system that uses 100% safe and renewable cooling-lubricating chemistries in much smaller quantities than used in conventional MQCL schemes.